A prior art powder reservoir of the kind as herein referred is described in U.S. Pat. No. 1,664,369. For obtaining a measured out admixture of the powder particles a feed screw driven by a motor is arranged in a cylindrically formed mixing chamber for feeding the powder particles that are discharged into the mixing chamber at the one end of the feed screw to an exit which is provided at an opposite end of the feed screw. At this opposite end compressed air is supplied into the mixing chamber for taking up the powder particles and transporting the mixture further along a connecting line with the handpiece.
The U.S. Pat. No. 2,696,049 discloses a mesh screen which is arranged for closing the discharge opening of a powder reservoir and for determining the dosage of the powder particles that are supplied via the screen to the mixing chamber for being mixed with a gaseous stream that is passed through the mixing chamber along the underside of the mesh screen. For supporting the discharge of the powder particles and for also avoiding any clogging of the screen the powder reservoir is arranged on a vibratory plate which is electromagnetically driven for oscillating the powder reservoir horizontally and vertically whereby the oscillations of the powder reservoir also contribute indirectly to move the powder particles towards the screen which closes the discharge opening of the powder reservoir.
The U.S. Pat. No. 4,482,322 discloses for same purposes an electromagnetically driven vibration device which supports a powder reservoir having a discharge opening which is normally closed by a diaphragm. The diaphragm is fixed along a rim portion and biased towards its closing position by a compression spring for supporting a pneumatic pressure compensation which is regulated for a mixing chamber which is provided underneath of discharge opening of the powder reservoir. The diaphragm separates the mixing chamber against a pressure compensation chamber and is transferred to a permanent opening position as soon as a pressure release via a branch line of a passage line for compressed air is regulated by means of a venting valve. In the opening position of the diaphragm the discharge of the powder particles from the powder reservoir will primarily be supported by the stream of compressed air which is directed over the upper surface of the diaphragm and stimulating a vortex motion inside of the mixing chamber The mixing chamber is designed by structural features as a specific vortex chamber into which the powder particles are discharged with the additional support of the vibration device which is switched-on in the opening position of the diaphragm.